Through tensile tests, longitudinal tensile properties of fiber reinforced polymer(FRP) bars after high temperature were studied. Test parameters include temperature, FRP bar diameter, FRP bar type and constant temperature time. The results show that, the stress strain relationship curves before and after high temperature are similar. The tensile strength and ultimate strain of FRP bars decline gradually as temperature rises. Within the range of test temperature, tensile modulus of elasticity is less affected by high temperature. With the decrease of FRP bars diameter, the tensile strength reduction ratio of BFRP bars appears to increase. The diameter of BFRP bars has no obvious influence on modulus of elasticity and ultimate strain. The high temperature resistance of BFRP bars is better than that of GFRP bars. When the temperature is at 270 ℃, FRP bars have finished most glass transition and thermal decomposition in the first half an hour. Finally, tensile strength reduction factor of FRP bars after high temperature was suggested to be under 300°C，when the temperature is higher than 300 ℃, BFRP bars and GFRP bars should not be used.